Admonish soldiers wanting to demonstrate (7)
I believe the answer is:
reprove
'admonish' is the definition.
('reprove' can be a synonym of 'admonish')
'soldiers wanting to demonstrate' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'wanting' means one lot of letters go next to another.
'to demonstrate' becomes 'prove' (proving is a kind of demonstrating).
're'+'prove'='REPROVE'
(Other definitions for reprove that I've seen before include "To remonstrate" , "tick off" , "Rebuke, censure" , "Criticise" , "Admonish" .)